1. Field of the Invention
The present invention is related to the field of electric wireline production logging tools. More specifically, the present invention is concerned with tools which measure the density of a fluid in a wellbore.
2. Discussion of the Relevant Art
Electric wireline production logging tools are used to determine volumes of various fluids entering a wellbore penetrating each formations, and the points of envy of the fluids within the wellbore. The fluids can include various quantities of gas, oil and water. By determining the points of entry and the volumes of the fluids entering the wellbore at each entry point, the wellbore operator may be able to take appropriate action in the event that fluids not desired to be produced from the wellbore appear in the total volume of produced fluid which reaches the earth's surface.
Various production logging instruments have been devised for determining fluid volume and points of fluid end, Production logging instruments include fluid density tools which measure the density of the fluid filling the wellbore. The fluid density tool typically transmits signals to the earth's surface over an armored electrical cable which correspond to the density of the fluid in the wellbore measured at a plurality of depths within the wellbore.
One type of fluid density tool enables determining the fluid density by measuring a differential pressure in the wellbore between at least two spaced apart locations along the tool. If the spaced apart locations are vertically separated, the differential pressure which exist between the spaced apart locations can be related to fluid density by the expression: ##EQU1## where .rho. is the fluid density; g is the local acceleration due to earth's gravity; .DELTA.P is the differential pressure; and .DELTA.h is the vertical separation between the spaced apart locations along the tool.
A differential pressure fluid density tool is known in the art. U.S. Pat. No. 3,616,688, issued to Bonnet et al, discloses an apparatus for determining fluid density by measuring differential fluid pressure between two spaced apart locations along the tool.
The differential pressure fluid density tool disclosed in the Bonnet patent has several drawbacks. Accurate measurement of differential pressure requires a differential pressure transducer which is sensitive to very small differences in pressure, on the order of 0.01 psi, applied across two pressure inputs to the transducer. Differential pressure transducers having this level of sensitivity are subject to destructive failure at relatively small values of differential pressure, on the order of 20 psi, applied across the transducer inputs. Destructive amounts of differential pressure can be encountered by the transducer for example, when the tool is inserted into the wellbore through a wellhead valve system attached to the top of the wellbore.
Another drawback to the differential pressure fluid density tool disclosed in the Bonnet patent is that the vertical separation between the spaced apart locations along the tool must be precisely known in order to determine the fluid density from the differential pressure measurement. Some wellbores are drilled directionally, and therefore have some inclination from vertical. If the tool is disposed in a non-vertical wellbore, the vertical separation between the spaced apart locations along the tool will not exactly correspond to the linear distance between the spaced apart locations. A measurement of the inclination from vertical, such as that provided by a directional survey, can be used to calculate mathematically the vertical separation between the spaced apart locations in a non-vertical wellbore, but directional surveys are frequently conducted at intervals as long as ninety feet within the wellbore. The actual inclination at some intervals of the wellbore may not be precisely determinable using a directional survey. Inaccuracy in determining inclination can cause inaccuracy in calculating the exact vertical separation between the spaced apart locations if the wellbore is highly deviated.
Another limitation of the fluid density tool disclosed in the Bonnet patent is that the differential pressure transducer is subject to variations in calibration when the transducer is exposed to different temperatures and absolute pressures. A typical wellbore has wide variations of pressure and temperature between the earth's surface and depths within the wellbore where the tool is used. In the tool disclosed in the Bonnet patent, the only calibration is to adjust the transducer output to indicate zero differential pressure with the tool lying flat, in air, at the earth's surface. The tool known in the art has no means for calibrating the transducer output to compensate for variations which may be induced by temperature and hydrostatic pressure.
It is an object of the present invention to provide a differential pressure fluid density tool which is selectively insensitive to differential pressure to protect the transducer from destructive failure.
It is a further object of the present invention to provide a differential pressure fluid density tool which makes differential pressure measurements which need not be corrected to account for inclination of the wellbore.
It is still a further object of the present invention to provide a differential pressure fluid density tool which has an internal means for compensating calibration changes in the differential pressure transducer measurement while the tool is disposed within the wellbore.